It is not uncommon to communicate data over mobile radio. Generally, the areas covered by the mobiles is significantly greater than the area that can be serviced from a single base station and thus a plurality of base stations have to be arranged in a group with a single or plurality of frequency assignments and the mobile must be capable of transferring communication from one base station to another as the mobile travels beyond the range of the one base station. This is obtained by distributing base stations throughout the total area to be serviced with each base station providing local coverage and a central control unit which is remotely linked to these base stations controlling the operations and transmission of the base stations. As above indicated, these base stations are selectively linked to the mobile stations within their service area.
Various systems have been devised to permit the mobile to transfer from one base station to another as the signal from the one base station is no longer acceptable. A typical such system normally utilizes the base stations to monitor the signal strength of the signal received from the mobile with which it is communicating and when the signal strength drops below a threshold level the mobile is automatically transferred to a second station receiving a significantly stronger signal from the mobile. U.S. Pat. No. 4,308,429 discloses a similar system for voice transmission.
U.S. Pat. No. 2,694,141 describes a system designed for terrestrial telephony requirements and initiates a channel reselection on the basis of a high noise to signal ratio, thus while it does in a sense monitor quality of the transmitted signal it would not be effective for data transmission between base stations and a mobile unit.
U.S. Pat. No. 4,419,766 relates to a system designed for air-borne telephony requirements and maintains cooperation with a single transmitter during the entire transmission, however its selects the transmitter with which it will communicate for the call by the air-borne unit assessing the signal strength and doppler shift in the signals from the transmitters.
U.S. Pat. No. 4,598,042 also describes a system for telephony requirements i.e. voice transmission and makes a transfer selection based on received signal strength at the mobile. This system does not accommodate base transmitters that are continuously keyed and appears to be adapted to only single frequency use within all areas operation.
It has been found that for data transmission systems the monitoring of signal strength and the use of signal strength to determine when a transfer is to be made is in itself inadequate.